The invention further relates to a cement clinker production plant comprising a clinker kiln to whose output-side end a clinker cooler is connected and to whose feed-side end a preheater, and optionally a calciner, are connected, wherein the preheater comprises at least one string of a plurality of cyclone suspension-type heat exchangers, through which the kiln exhaust gas is able to successively flow along a flow path and in which the raw meal is preheated in stages.
In cement clinker production, raw meal is preheated, completely dried, calcined, burned to clinker, and subsequently cooled. Plants operated according to this drying procedure comprise a preheater comprised of cyclone suspension-type heat exchangers, a calciner, a tertiary air duct, a rotary kiln, and a clinker cooler. The energy required for the material conversion in this plant is provided by supplying fuel to the rotary kiln and to the calciner. The air heated in the clinker cooler is returned partially to the rotary kiln as so-called secondary air and partially to the calciner as so-called tertiary air. The exhaust gases of the rotary kiln are conducted to the calciner through a kiln feed chamber and a flow contraction provided thereabove, flow through the same, and are discharged into the preheater along with the exhaust gases produced in the calciner and consisting of smoke gas from the calciner fuel and CO2.
The preheater is comprised of one or several strings, and each string comprises several heat exchanger stages each formed by a cyclone suspension-type heat exchanger. The dry cement raw meal is charged into the vertical tube of the uppermost heat exchanger, travels through the heat exchanger stages from top to bottom, and is conducted into the calciner from the second-lowermost heat exchanger stage. In the calciner, the hot raw meal is almost completely deacidified and, together with the exhaust gas from the calciner, flows into the lowermost heat exchanger stage, is separated there, is charged into the kiln feed chamber, and reaches the rotary kiln as hot meal through the former. The hot meal is burned to clinker in the rotary kiln by a sintering process.
The thermal energy contained in the calciner exhaust gas, of about 1.4 normal m3/kg of clinker and 850 to 890° C. is stepwisely given off to the fresh raw meal in co-current heat exchange. With the number of heat exchanger stages increasing, the temperature of the exhaust gas will decrease, the thermal efficiency of the kiln plant will improve, and the heat exchanger tower will increase in size and costs. Typically, four to six such stages are built, the number of stages being primarily a function of the moisture contained in the raw material.
The useful heat contained in the exhaust gas from the clinker kiln and the calciner exceeds the absorptive power of the raw meal due to the usual quantitative ratio and the characteristic of the multi-stage heat exchange. The thermal energy available at the kiln gas exit from the preheater, therefore, still comprises a useful residual heat potential of about 1.5 normal m3/kg of clinker of 290 to >350° C. This can be further utilized for drying raw materials and fuels as well as for other purposes, e.g. the conversion into electricity, outside the thermal process.
The kiln exhaust gas is drawn through the heat exchanger stages by the aid of an induced draught ventilator. Since, in doing so, the total amount of kiln exhaust gas is drawn through all of the heat exchanger stages, the fluidic cross sections of the heat exchanger stages are to be dimensioned as large as possible in order to minimize the pressure drop, and hence the power required by the air suction ventilator. This will, however, involve plant costs that are directly dependent on the size.